The present invention relates to a method of adjusting the polarization intensity for high numerical apertures (NA), and more particularly, to a method of adjusting the intensity ratio of an S wave polarization to a P wave polarization after penetrating a photoresist layer to a predetermined depth.
The photolithography process is very important in the field of semiconductor manufacturing. The definition of each thin film, the interconnects, the doped areas, etc. are controlled by the photolithography process. The photolithography process is very complicated, including the coating of a photoresist layer, an exposure process, a development process, etc. Since the patterns of the mask must be transferred to the semiconductor wafer, the quality of the photolithography process is decisive to the product yield.
FIG. 1 is a schematic diagram of an exposure light source emitting light. During a photolithography process, an incident linearly polarized light 10 penetrates a photoresist layer 12 through a reduction lens (not shown) of high numerical aperture (NA), and has an incident angle xcex8. The incident linearly polarized light 10 is divided into an S wave polarization 14 and a P wave polarization 16, which are orthogonal. The S wave polarization 14 has an electric field 18 and a magnetic field 20, and the P wave polarization has an electric field 22 and a magnetic field 24 wherein the electric field 18 is perpendicular to the electric field 22.
FIG. 2 shows the transmission coefficients of the S wave polarization and the P wave polarization for the photoresist layer according to the prior art. As shown in FIG. 2, using NA=sin xcex8 (where xcex8 is the incident angle), when the NA of the reduction lens is more than 0.7, the incident angle xcex8 is more than sinxe2x88x9210.7, and the S wave polarization 14 has a transmission coefficient for the photoresist layer 12 that is different from the P wave polarization.
Since the S wave polarization 14 and the P wave polarization 16 have different transmission coefficients for the photoresist layer 12, the intensity ratio of the S wave polarization 14 to the P wave polarization after penetrating the photoresist layer 12 is not 1:1. Following this, due to the intensity ratio not being 1:1, the exposure effect along the perpendicular direction and the parallel direction of the patterns is not equal. This then causes astigmatism.
Astigmatism affects the fidelity of the parallel direction and the perpendicular direction of the patterns, and affects the exposure process greatly. Moreover, all the photolithography processes for high NA have similar problems.
It is therefore an objective of the present invention to provide a method of improving astigmatism of the photoresist layer.
It is another objective of the present invention to provide a method of adjusting the intensity of the polarization for high NA.
The preferred embodiment of the present invention provides an exposure system comprising a light source to provide light, a mask comprising a mask pattern, the light capable of passing through the mask pattern, a photoresist layer comprising an optically active component, a reduction lens system adapted to accept light passing through the mask to form an image in the photoresist layer, and a linear polarizer positioned between the light source and the photoresist layer to linearly polarize the light from the light source. The optically active component divides linearly polarized light into an intensity-adjusted S wave polarization and an intensity-adjusted P wave polarization. The proportion of the intensity of the S wave polarization to the P wave polarization is effectively 1:1 after penetrating the photoresist layer to a predetermined depth.
It is an advantage of the present invention that it can improve astigmatism of all exposure processes having high NA.
These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment, which is illustrated in the various figures and drawings.